This description relates to reverse link power control.
ACRONYMS AND ABBREVIATIONS1x-EVDOEvolution Data only, CDMA2000 family standardfor high speed data only wireless internet accessANAccess NetworkAPIApplication Programmable InterfaceASICApplication Specific Integrated CircuitATAccess TerminalBIO-SCBasic Input/Output - System ControllerCDMACode-Division Multiple AccessCPUCentral Processing UnitCSM5500 ASICQualcomm Inc. modem ASICCSM5500 DriversQualcomm Inc. modem ASIC Driver and APIFCSFrame Check SequenceFERFrame Error RateFLMForward Link ModemIPInternet ProtocolPCTPower Control ThresholdPCTRNiPower Control Threshold computed at ith RNPCTRNCPower Control Threshold computed at RNCRANRadio Access NetworkRLReverse Link or uplink - from mobile to base station.RLILPCReverse Link Inner-Loop Power ControlRLMReverse Link ModemRLOLPCReverse Link Outer-Loop Power ControlRLOLPC-RNPower Control Algorithm running on RNRLOLPC-RNCPower Control Algorithm running on RNCRNRadio Node or Base StationRN-BIO-SCRadio Node BIO-SC Card or moduleRNCRadio Network ControllerRNSMRadio Network Serving ModuleRPCReverse Power ControlRTCHMOReverse Traffic Channel MAC ObjectSDUSelection and Distribution UnitSINRSignal-to-Interference Ratio (Eb/It)
Capacity of a cellular system represents the total number of mobile users (access terminals or ATs) that can be supported by the system. Capacity can be an important factor for cellular service providers, since it directly impacts revenue. CDMA wireless communications systems offer improved capacity and reliable communications for cellular and PCS systems.
In a CDMA system, each AT transmit signal utilizes a different pseudo random sequence signal that appears as noise to other ATs. This enables many ATs to transmit on the same frequency. However, each AT's transmitted signal contributes to interference to the transmitted signal of all other users. Thus, the total number of users supported by the system is limited by interference. Therefore, reducing the amount of interference in a CDMA wireless communications system increases capacity.
A typical problem in a CDMA cellular environment is the near/far problem. This entails the scenario where the transmit power of an AT near the RN may drown out an AT which is far from the RN. This is effectively mitigated by controlling the transmit power of each AT via power control scheme implemented by the access network (AN). AN continuously commands each AT to increase or decrease its transmit power to keep them all transmitting at the minimal power required to achieved the configured error rate for the operating data rate and maintain the overall balance of the power while reducing the interference in the area of coverage.
In a CDMA 1x-EVDO system (see e.g., CDMA2000 High Data Rate Packet Data Air Interface Specification, 3GPP2 C.S0024, Version 4.0, Oct. 25, 2002), the reverse link operates in CDMA and hence reverse link power control is needed. The reverse link power control comprises of an open-loop power control (also called autonomous power control) and closed-loop power control. Open-loop power control is implemented in an AT, based on the received pilot-power of an RN. Closed-loop power control includes inner loop power control and outer loop power control, both of which are performed by the access network. Typical operation of a closed loop power control can be found in textbooks (see e.g., Vijay K. Garg, IS-95 CDMA and CDMA2000 Cellular/PCS Systems Implementation, Chapter 10, Prentice Hall, 1999, R. Steele. Mobile Radio Communications. Pentech Press, London, England, 1992, and Rashid A. Attar and Eduardo Esteves, A Reverse Link Outer-Loop Power Control Algorithm for CDMA2000 1xEV Systems, Proceedings of ICC, April 2002). Also, additional details can be found in e.g., U.S. Pat. No. 6,633,552, titled Method And Apparatus For Determining The Closed Loop Power Control Set Point In A Wireless Packet Data Communication System, and issued on Oct. 14, 2003, U.S. Pat. No. 6,507,744, titled Outer Loop Power Control Method During A Soft Handoff Operation, and issued on Jan. 14, 2003, and U.S. Pat. No. 5,884,187, titled Outer Loop Power Control Method During A Soft Handoff Operation, and issued on Mar. 16, 1999. A typical implementation is now described.
FIG. 1 illustrates a system 100 implementing the basic closed loop power control operation. In closed loop power control, power adjustment is done at an AT 105 in accordance with the power control commands received from an RN 110 (also referred to as a base station 110). RN 110 sends up/down commands to each active AT (e.g., 105) to ensure that the AT transmit signal is received at the RN 110 at the lowest possible power required for the RN 110 to receive the data correctly at the operating rate.
In a reverse link inner-loop power control (RLILPC) mechanism 115, the reverse link signal to the interference-noise ratio (SINR) is continuously and frequently measured at a modem receiver of RN110. These frequent measurements track rapid channel variations of the link between the AT 105 and the RN 110 and facilitate accurate power control even when the AT 105 is in a deep fade. This measured of SINR is compared to a threshold value called ‘power control threshold’ (PCT). If the measured value is greater than PCTmax (=PCT+PCTDelta), the RPC bit is cleared. If the measured value is less than PCTmin (=PCT-PCTDelta), RPC bit is set. PCTDelta is a small value that provides an interval around the PCT. If the PCT is within this interval, the RPC bit status is unchanged from the previous value. Setting the RPC bits (‘up decisions’) commands AT 105 to increase its transmit power by a pre-determined step size, say ‘x’ dB. Clearing RPC bits (‘down decisions’) commands the AT 105 to decrease its transmit power by ‘x’ dB. The step size is negotiated a priori between RN 110 and AT 105.
Frame Error Rate (FER) is defined as a ratio of the bad frames to the total number of frames received by the RN 110. A frame with correct physical layer frame check sequence (FCS) is defined to be a good frame. In 1x-EVDO, the physical layer cyclic redundancy code (CRC) can be used to determine good or bad frames. In a reverse link closed outer-loop power control (RLOLPC) algorithm 120, the PCT is adaptively adjusted such that the configured target FER is achieved and maintained for the duration of the connection. (A target reverse link FER of 1% is considered typical for wireless networks). The RLOLPC algorithm 120 is implemented in a RNC 125.
It should be noted that there is another parameter beside FCS that is used in the voice application in CDMA system. This parameter is called the quality metric, which is an indication of how “bad” the bad frame is. For voice, it may be beneficial to play out a bad packet in order to maintain the perception of a good voice quality. Therefore, even the bad packets are still sent to the RNC 125 from the RN 110 with the marking for a correct FCS and a quality metric. It's up to the RNC 125 to determine if the quality metric meets the criteria for the packet to be used even when the FCS is incorrect.
Typical operation of the RLOLPC algorithm 120 is described now. Upon reception of a RL frame with bad FCS, PCT is increased by a pre-set large value (e.g., 0.5 dB), which is termed a good frame PCT Delta. Upon reception of a RL frame with good FCS, PCT is decreased by a pre-set small value (e.g., 0.5 dB), which is termed a bad frame PCT Delta. Given the values of RL FER and the good frame PCT Delta, the bad frame PCT Delta value is computed as follows:
Bad Frame PCT Delta=Good Frame PCT Delta (1−RL FER)/RL FER (Note that this same equation can be used to compute the good frame PCT Delta given the values of RL FER and the bad frame PCT Delta.) Before a connection establishment, or if there is no data on a RL, the PCT is set to a pre-set high value to facilitate rapid reverse link acquisition. A new value of the PCT is computed upon reception of each good/bad RL frame and an updated PCT is input into the RN modem receiver and to the RLILPC algorithm 115.
FIG. 2 illustrates an example system 200, in which AT 105 is in a L-way soft hand-off (i.e., AT 105 is communicating with L RNs, e.g., RN1, 110, RN2 205, and RNL 210, at the same time). The selection and distribution unit (SDU) (not shown) at the RNC 125 determines which received frame from all the different ‘legs’ should be used. In addition it determines if correct or incorrect received frame indication needs to be send to the RLOLPC algorithm 120 on each frame boundary. The RLOLPC algorithm 120 uses this information to compute the overall PCT for the AT 105. This PCT value is sent to all L RNs involved in the soft hand-off.